Cognac is what we call in french an "AOC" (for "Appellation d'Origine Contrôlée"), like "Bordeaux" or "Champagne", whiwh means that you can only produce cognac in the delimited area of Cognac. This AOC was set by three main texts. The first one is a decree of 1909 that delimited the area of production of Cognac.
Then in 1936, a second decree set the rules of production of the cognac, regarding the winemaking, the distillation and the ageing. Finally, in 1938, a third decree officialy established the existence of six distinctives growth areas, according to their geological specificities that give different eaux-de-vies with specific flavors and maturation's ability.

If you want to know more about the delimited area of Cognac, you can go on the BNIC page, the professionnal body that cares about the defense of the cognac's interests and its promotion : www.cognac.fr